Transformation of cultured fibroblasts produces drastic changes in cell shape, adhesiveness, surface morphology, and agglutination by plant lectins. The changes that occur in the distribution of microfilamentous contractile proteins after transformation were found to be the result of changes in cell shape as a result of primary changes in cell to substratum adhesiveness, and not directly related to changes in growth properties. Changes in lectin agglutination after transformation have been linked to changes in surface morphology (microvilli) which are a direct result of cell shape changes. The most significant change in growth properties, anchorage dependence, has been narrowed to an effect on the exit from the G1 phase of the cell cycle. A new method of ultrastructural immunocytochemical localization has been developed which will help in the understanding of the distribution and function of cell proteins. BIBLIOGRAPHIC REFERENCES: Pouyssegur, J., Willingham, M.C., and Pastan, I.: Role of Cell Surface Carbohydrates and Proteins in Cell Behavior: Studies on the Biochemical Reversion of an N-acetylglucosamine-deficient Fibroblast Mutant, Proc. Natl. Acad. Sci. USA 74: 243-247, 1977. Willingham, M.C., Yamada, K.M., Yamada, S.S., Pouyssegur, J., and Pastan, I.: Microfilament Bundles and Cell Shape are Related to Adhesiveness to Substratum and are Dissociable from Growth Control in Cultured Fibroblasts. Cell 10: 375-380, 1977.